Adopting C17

From: Minsoo Choo <minsoochoo0122_at_proton.me>
Date: Sun, 28 Jan 2024 05:44:20 UTC
Hi everyone

There are three opened revisions related to adopting C17(gnu17 precisely) for kernel and user base. If there is any questions or suggestions, please let me know.

Revisions:
gnu17 in user base: https://reviews.freebsd.org/D43237
gnu17 in kernel: https://reviews.freebsd.org/D43239
Removing pre C17 support: https://reviews.freebsd.org/D43254